Patrick -
The primary Salix floral visitors in the temperate east tend to be andrenids
(mining
bees), along with halictids and a variety of flies. Willows tend to
attract short-tongued, rather than long-tongued bees, and many Bombus fall into
the latter category. Also, since Bombus need some time in the spring for
queens to find nest sites and start
building a colony, many willows might be done flowering before large
numbers of foraging Bombus workers were out and about.
The consensus seems to be that anemophily is an ancestral state shared by
Populus and Salix, and that entomophily has evolved (or re-evolved, depending
on your views of plant evolutionary history) in Salix.
Some willow species are entirely insect-pollinated, some almost
entirely wind-pollinated, but the really interesting ones are the
species in which the ratios of wind to insect pollination can vary as
much 50% or more. The following papers are informative on the topic.
I'm not up to date on this literature, so I'm sure something more has
been done since this, but it provides a starting point.
Peeters, L., Totland O. 1999. Wind to insect pollination ratios and floral
traits in five alpine Salix species. Canadian Journal of Botany 77(4): 556-563
Tamura S., Kudo G. 2000. Wind pollination and insect pollination of two
temperate willow species, Salix miyabeana and Salix sachalinensis. Vegetatio
147(2): 185-192.
Abstract:
Pollination
systems of Salix miyabeana and Salix sachalinensis were studied at a
riverside in northern Japan in order to measure the balance of wind
pollination and insect pollination. In 1996, 19 clones of each species
were selected, and seed-set success by a net-bagging (to exclude insect
visitation) and an artificial pollination (to remove pollen limitation)
were compared to by natural pollination. For S. miyabeana, the same
experiment was repeated in two populations in 1997. Proportion of seed
set through wind pollination dominated in both species in this study.
Pollen limitation was common under natural conditions, and
S. sachalinensis relied more on insect pollination for seed production
than S. miyabeana. Meteorological factors such as precipitation and
hours of sunshine during the flowering season influenced the potential
reproductive activity of the willow between years. In the wet and
cloudy spring of 1996, clones which obtained high seed set depended
more on insect pollination for both species, whereas in the dry and
sunny spring of 1997, such clones depended more on wind pollination for
S. miyabeana. Because the efficiency of wind pollination seemed to be
more sensitive to fluctuating weather conditions than insect
pollination, insect pollination was considered to play an assurance
role for seed production in these willows.
Matt
